Large effects of extreme rainfall event on dissolved carbon in a subtropical karst river in South China

Abstract

The dynamics of dissolved organic and inorganic carbon (DOC and DIC) in subtropical rivers within karstic catchment exhibit pronounced sensitivity to rainfall intensity, particularly under extreme hydrologic forcing. Because the South China Sea receives 6.2–10.3 % of global riverine carbon fluxes, resolving the mechanistic links between rainfall regimes and carbon dynamics in this climate-sensitive region is critical for constraining global land-ocean carbon budgets. Through sampling across dry seasons, monsoon periods, and typhoon events in a representative South China karstic catchment, we demonstrate three rainfall-intensity-dependent carbon pathways. Extreme rainfall triggered rapid terrestrial-aquatic carbon transporting, elevating DOC concentrations by 40–174 % (mean: 13.4 mg/L). Fresh, bioavailable dissolved organic matter (DOM) dominated these pulses, evidenced by fluorescence index (FI: 1.98–2.16), biological index (BIX: 0.71–1.03), and higher protein-like components (41–63 %). Concurrently, shortened water residence times suppressed carbonate weathering, yielding ¹³C-depleted DIC (δ¹³C-DIC: −24.9 ‰ to −18.8 ‰, HCO₃⁻: 33.8–67.7 mg/L) in riverine systems. Sustained moderate rainfall facilitated complete carbonate dissolution (δ¹³C-DIC enrichment to a mean value of −13.6 ‰, HCO₃⁻: 38.0–65.7 mg/L) through prolonged water-rock interaction. Microbial metabolism degraded labile DOC (FI: 1.69–1.81; protein-like component: 0–48.6 %), leaving recalcitrant humic substrates (SUVA₂₅₄: 2.3–7.2 L/mgC/m) and lower DOC concentrations (mean: 4.9 mg/L). Dry-season baseflow exhibited algal-mediated carbon cycling: photosynthetic DIC uptake enriched δ¹³C-DIC (−16.7 ‰ to −8.3 ‰), while autochthonous DOC production maintained moderate levels (mean: 9.5 mg/L), evidenced by a higher slope ratio (S_R: 0.88–1.46). These findings establish a rainfall intensity-carbon speciation matrix, revealing that hydrologic extremes prioritize rapid DOC transfer over DIC generation, moderate rains maximize weathering-derived DIC, and baseflow sustains aquatic carbon release. This study highlights the sensitive response of DOC and DIC to rainfall events in subtropical rivers, providing new insights into the processes governing terrestrial-to-aquatic carbon dynamics in subtropical karst systems.